The type-1 human immunodeficiency virus (HIV-1) has been implicated as the primary cause of the slowly degenerate disease of the immune system termed acquired immune deficiency syndrome (AIDS) (Barre-Sinoussi, F. et al., 1983 Science 220:868-70; Gallo, R. et al. 1984, Science 224:500-3). Infection of the CD4+ subclass of T-lymphocytes with the HIV-1 virus leads to depletion of this essential lymphocyte subclass which inevitably leads to opportunistic infections, neurological disease, neoplastic growth and eventually death. HIV-1 infection and HIV-1 associated diseases represent a major health problem and considerable attention is currently being directed towards the successful design of effective therapeutics.
HIV-1 is a member of the lentivirus family of retroviruses (Teich, N. et al., 1984 In RNA Tumor Viruses ed. R. Weiss, N. Teich, H. Varmus, J. Coffin CSH Press, pp. 949-56). The life cycle of HIV-1 is characterized by a period of proviral latency followed by active replication of the virus. The primary cellular target for the infectious HIV-1 virus is the CD4 subset of human T-lymphocytes. Targeting of the virus to the CD4 subset of cells is due to the fact that the CD4 cell surface protein acts as the cellular receptor for the HIV-1 virus (Dalgleish, A. et al., 2984, Nature 312:763-67; Klatzmann et al. 1984, Nature 312:767-68; Maddon et al. 1986 Cell 47:333-48).
After binding to the cell surface, the HIV-1 virion becomes internalized, and once inside the cell, the viral life cycle begins by conversion of the RNA genome into linear DNA molecules. This process is dependent on the action of the virally encoded reverse transcriptase. Following replication of the viral genome, the linear DNA molecule integrates into the host genome through the action of the viral integrase protein, thus establishing the proviral form of HIV-1.
During the early phase of proviral expression, transcription of the viral genome results in expression of regulatory proteins such as Tat, Nef and Rev. Transcriptional activation of the proviral DNA is mediated through the viral 5' LTR sequences (long terminal repeats). The initial low level of viral transcription is dramatically increased by the HIV encoded transactivator protein termed tat (transactivator protein) (Cullen, B. R. et al. 1989, Cell 58:423-26). The Rev protein promotes the transition from the early phase expression of regulatory proteins to late phase expression of structural proteins. Assembly of newly synthesized viral particles is followed by budding of virus particles from the cell membrane allowing the virus to infect new cells.
The HIV-1 virus is capable of establishing a latent state of infection for prolonged periods of time. Individuals infected with the human immunodeficiency virus may remain clinically healthy for long periods of time, with the estimated average length of the asymptomatic period between primary HIV infection and the progression to AIDS and increase in viral replication being approximately 8 to 10 years. Several possibilities have been proposed to explain the maintenance of the low levels of viral replication during this period of latency. It is generally believed that the humoral immune response to HIV-1 is not sufficiently protective against progression of the disease and attention has, therefore, turned to the possibility that the T-lymphocyte population of cells may directly inhibit HIV-1 replication.
A number of groups have recently noted that the CD8+subset of T-lymphocytes have the ability to inhibit the replication of HIV-1 in vitro (Walker, C. M. et al., 1989, Cellular Immunology 119:470-475; Kannagi, M. et al. 1990, J. Virology 64:3399-3406; Walker, C. M. et al., 1991 J. Virology 65:5921-5927). For example, addition of CD8 cells to naturally HIV-1 infected CD4 cell cultures was found to inhibit the replication of HIV-1 in the infected cultures in a dose dependent manner. (Ref. supra). Furthermore, the inhibitory effect is not dependent on cell-cell contact as an inhibitory effect is observed across a semi-permeable membrane suggesting that the CD8 suppressor activity is a soluble inhibitor of HIV-1 replication. (Ref. supra). To date, the molecular identity of the CD8 suppressor molecule, or a combination of factors, as well as the mechanism by which it exerts its antiviral effect remains undefined.